Many attempts have been made to construct a practical electric current sensor utilizing the Faraday effect. In a first class of such sensors, an optical fiber is formed into a coil of several turns, and the electric current flows in a conductor along the axis of the coil. The magnetic field produced by the current flow interacts with the optical fiber to rotate the direction of polarization of linearly polarized light launched into the fiber. Stress birefringence in the optical fiber also causes the state of polarization of the linearly polarized light propagating within the fiber to change. Light exiting from the fiber has its state of polarization changed both by the stress birefringence and by the Faraday effect. It is necessary to separate the polarization rotation produced by the Faraday effect from the unwanted changes in polarization state produced by other effects in order to measure the electric current flow. An additional complicating factor is that the stress birefringence of the fiber depends upon the temperature of the fiber. Thus, if the temperature of the fiber changes during time, then the stress birefringence will change. The variation of the stress birefringence with temperature adds an additional level of complexity to the attempt to separate the rotation of the polarization due to the Faraday effect and the distortions in the state of polarization due to other factors. The complexity is particularly troublesome when the disturbing effects due to stress birefringence change with time as the temperature of the fiber changes.
A second class of Faraday effect current sensors utilize solid blocks of transparent material, where the blocks of material are penetrated by a magnetic field produced by the current flow. Again, birefringence in the block of optical material introduces changes in the state of polarization of the light propagating in the solid transparent block and these changes must be separated from the rotation produced by the Faraday effect. In the event that the block material exhibits stress birefringence, the stress birefringence may be due to stresses produced in the glass when the glass was annealed, or may be induced in the glass by mechanical forces used to clamp the glass in position. Also, any stress birefringence in the glass is usually dependent upon the temperature of the glass. For example, if the glass is not at a uniform temperature, that is if one side of the glass is at a slightly different temperature from another side of the glass, this temperature gradient will set up stresses within the glass which may induce stress birefringence. So again, the separation of the rotation of the polarization direction induced by Faraday effect from changes in the state of polarization induced by other birefringent processes such as stress birefringence is an unsolved problem.